The present invention relates to a method of supplementing the catalyst in continuous hydroformylation during ongoing operation.
The continuous hydroformylation of olefins using a rhodium catalyst system is a well-known mature process for the commercial preparation of oxo aldehydes and their hydrogenation products, the oxo alcohols. The process is described in numerous patent documents, e.g. in U.S. Pat. No. 3,527,809, U.S. Pat. No. 4,148,830, U.S. Pat. No. 4,247,486 and U.S. Pat. No. 4,247,468.
In continuous operation, the activity of the catalyst system decreases over the course of time. To maintain the original activity of the catalyst system and the productivity of the reactor system, the catalyst constituents which have become inactive have to be replaced at intervals by addition of fresh transition metal compounds and/or fresh ligands from which further amounts of the active catalyst species are formed under the hydroformylation conditions. Since the hydroformylation is a strongly exothermic reaction, a runaway reaction as a result of the increased catalyst activity has to be avoided at all costs.
At present, when fresh transition metal compounds are added, e.g. in the case of addition of rhodium acetate, the reactor throughput, the amount of starting olefin fed in, the reactor temperature and/or the level of liquid in the reactor is/are reduced before the transition metal compound is added within a short period of time of typically from 2 to 5 hours. This procedure enables the evolution of heat associated with the addition and the temperature increase associated therewith in the reactor to be controlled readily and a runaway reaction to be effectively prevented.
However, this procedure has the disadvantage that the capacity of the hydroformylation plant is not fully utilized during the supplementation of the transition metal compound, which leads to corresponding production losses.
It is an object of the present invention to provide an improved process for preparing saturated C3-C20-alcohols from C3-C20-aldehydes, in which safe supplementation of the transition metal compound is combined with very low production losses.
It has now surprisingly been found that the transition metal source can be supplemented in continuous hydroformylation without a greatly reduced conversion or a greatly reduced yield having to be accepted when the rate of introduction of the transition metal source into the reaction zone is controlled as a function of the space-time yield. In this way, it is possible, at essentially full loading of the reactor, to keep the activity of the catalyst in the desired target range, with the addition of the transition metal source bringing about only a marginal temperature increase in the reactor which can readily be controlled by engineering means.